Paint coatings are used as surface protective coatings applied to substrates as a continuous film for the purpose of decorative appearance as well as protection of the substrate. Thermosetting paint coatings can be fluid liquid coatings or fusible solid compositions adapted to flow out and form a continuous paint film when heat cured. A paint coating ordinarily comprises an organic binder, pigments, and various additives. The polymeric binder acts as a vehicle for uniformly mixing the pigments and other components and functioning as a binder for the pigments in the cured or hardened paint film. The binder imparts rheological properties to the paint coating and provides adhesion of the paint film to the substrate.
Powder paints can be thermosetting or thermoplastic clear or pigmented paint coatings and ordinarily are factory applied coatings and cured under heat. Powder paint comprises dry particles and typically are compounded by dry blending of components followed by melt extrusion grinding, and screening, to form a dry free-flowing powder. Powder paints are applied to a substrate as a powder but adapted to flow out on the substrate upon heat curing of the powder. The pigments may be organic or inorganic and functionally contribute to opacity and color in addition to durability and hardness, although some powder paint coatings contain little or no opacifying pigments and are described as clear coatings. Polymers for powder coatings ordinarily are solvent-free and non-sintering solids at room temperature which must resist fusing in the container and are considered stable if sintering does not occur at 40.degree. C. for an extended length of time. Impact fusion of a powder coating formula in automated application systems is a further problem to avoid. Problems with stability and impact fusion can be avoided by the tack temperature of a polymer being at least 150.degree. F. Tack temperature (Tt) is a measure of the sinter point of a powdered polymer in one minute of time. A useful tack temperature range of 175.degree. F. to 190.degree. F. is preferred for polymers used in most powder formulations.
In liquid paint formulations, soft (low Tg) polymeric components are often included in polymer synthesis to impart desirable properties such as flexibility, impact, and flow to the coating. However, soft low Tg components are either avoided or kept to a minimum in powder coating polymers, due to stability and impact fusion problems. For a coating technology to be feasible for powder coatings, the resin system must be a solid at room temperature and must soften at the extrusion temperature. The resin viscosity must also allow for adequate mixing and dispersion of pigments and additives during the residence time in the extruder, which enables a homogeneous melt-mixed product to be produced. The extrudate must be friable to enable grinding of the extrudate and flake to normal mean particle size distributions of about 25-40.mu.. Powder paint must apply well by a corona discharge gun and must electrostatically adhere to a metal part. At the bake temperature, the powder must melt and flow to produce a smooth, glossy finish and needs to cure at temperatures of about 300.degree. F. to 400.degree. F. Physical and mechanical properties such as gloss, hardness, flexibility, solvent resistance and corrosion resistance are highly desirable.
Polyester and acrylic polymers are polymers commonly used in powder coatings formulations. Polyesters usually are terephthalic acid based, high molecular weight, highly linear, amorphous, high melting polymers. Useful acrylics are high Tg and high melting polymers with hydroxyl or glycidyl functionality for crosslinking (curing) of the polymer. Acrylic/polyester blends are desirable for powder paints because straight acrylic systems tend to produce weatherable hard coatings, but unfortunately, lack the flexibility required for many end-uses. Polyester coatings are flexible and tough but, in general, provide inferior weatherability. In liquid coating technologies, the two polymer types mixed with organic solvent can be mixed to form a blend of acrylic polymer with a polyester polymer in the coating formulation to obtain both weatherability and flexibility. However, blending of acrylic and polyester polymers is not easily achieved in powder coatings inasmuch as the lack of solvent raises the free energy of mixing to such an extent that polymer incompatibility becomes a major obstacle and mixing of an acrylic polymer with a polyester polymer is not easily attained.
It now has been found that a thermosetting powder coating composition exhibiting excellent blend of physical properties such as appearance, flexibility/hardness, solvent resistance, corrosion resistance and exterior durability based on a polyester-graft-acrylic copolymer can be produced. The polyester-graft-acrylic produces compatibility with low melting crystalline polyesters to provide a coating which can be heat cured and crosslinked. The present invention utilizes a grafting technique to compatibilize acrylic polymer chains with polyester polymer chains. The cure mechanism can include a latent catalyst which gives the coating time to flow, wherein the catalyst activates the crosslinking cures with the polyester-graft-acrylic resin producing a smooth, flexible film. The present invention utilizes the uniqueness of polyester-graft-acrylic copolymers in combination with low molecular weight crystalline polyester compounds in powder coatings, which promotes sufficient compatibility between the polyester polymer chains and the acrylic polymer chains in a blend of otherwise incompatible polymers. In addition, the physical integrity properties of powder coatings are considerably enhanced. For instance, the powder paints of this invention exhibit improved physical properties, excellent melt-flow rheology/smoother appearance, higher flexibility and impact resistance, as well as good overall coating performance, i.e., gloss, color, hardness. The process of this invention provides improved compatible polymeric blends with useful tack temperatures in the range of 160.degree.-190.degree. F. along with desirable storage stability. Early attempts at blending the polyester polymers with acrylic polymers were unsuccessful due to either incompatibility or processing difficulties associated with widely varied viscosities, but these problems are overcome by this invention. Compatibility and eutectic melt problems have been eliminated or minimized to commercially acceptable levels for both powder processing (extruding/flaking/grinding) and commercial storage of the finished powder. These and other advantages of this invention will become more apparent from the detailed description of the invention.